Filters for removing polluting components from liquid and gas are known from the prior art. Filtration is usually understood as a process of separating dispersed particles, and some times solutes too, from a fluid by drawing the fluid across a porous medium. The invention described here is in this respect not a true filtration in that the fluid is not drawn across the porous medium, but flows along it without crossing it.
Traditional filters suffer from the dilemma that on one hand they need to have an open structure in order to retain a certain hydraulic conductivity, but on the other hand the structure must not be too open in order to retain dispersed particles and solutes. Traditional filters suffer from gradually falling hydraulic conductivity (pressure drop across the filter) as more and more particles are trapped within the filter medium. Clogging rather than use-up of filter capacity often determines the frequency of filter regeneration or replacement.
One type of prior art filters is based on a layered structure with different pore sizes and sorption capacities in the layers. The liquid or gas moves through the layers of these known filters sequentially, most often first through one or more macroporous layers to remove particulate material and then through one or more microporous layers having the capacity to absorb dissolved or dispersed material. The drawback of these filters is mainly that materials with low hydraulic capacity (and high absorptive capacity) cannot be used under practical circumstances, because the flow rate will be too low. Another drawback is that once the filtering capacity of the microporous layer in an area is exhausted, then the whole layer must be exchanged, as there is a risk the filter leaks pollutants.
Another prior art filter with two types of pores is one where micro- and macropores are mixed throughout the filter volume. Such filters consist of particles (e.g. perlite) or other macrostructures, which contain micropores. Macropores are formed between the particles. These filters generally may have a higher rate of flow through the filter, because the pollutants are trapped in the micropores, whereas the main flow is through the macropores. These filter units are inherently non-uniform, because the size of the macropores is determined by the dimensions of the particles/structures containing the micropores. Thus there is a risk that pores of too great size are created through the layers so that polluted liquid can run through the filter without coming into contact with the micropores. This may especially be the case along the outer boundaries of the filter layer, e.g. along the side of a container housing the particles/structures. In such a filter, it is not possible to place a sorbent of choice in the micropore region and the filter therefor cannot be adapted to remove a pollutant of choice. Neither is it possible to create a hydraulic conductivity of choice; it will always depend on the size of the filter particles/structures.
U.S. Pat. No. 6,080,307 (ABTECH INDUSTRIES) discloses a storm drain insert with a separate collection system for oil and other hydrocarbons. The filter-material consists of a copolymer of thermoplastic polymers such as styrene-butadiene-styrene. The filter-material according to one embodiment is formed as a cylindrical body with a centrally placed hole along its longitudinal body. The cylindrical bodies may have numerous fissures to increase the effective surface of the body.
U.S. Pat. No. 5,788,849 (HUTTER & PROBST) discloses a filter unit containing several filter components, wherein the filter components are placed in a horizontal orientation and thereby perpendicular to the direction of flow of the water. The hydraulic conductivity of such a filter unit is determined by the hydraulic conductivity of the filter component with the smallest pore size, thus the hydraulic conductivity can not be chosen freely. As filter materials with a high absorptive capacity inherently have a low hydraulic capacity a filter unit based on this design has a low hydraulic conductivity.
U.S. Pat. No. 5,776,567 (PACTEC INC.) discloses a multi-layer filter for separating solid and liquid wastes. In a preferred embodiment the filter includes four layers. The first layer may be a network of parallel strands. The second layer may be a fibrous mat, the third layer a netting like the first layer and the fourth layer a porous filter cloth. During use, water passes sequentially through the layers under the force of gravity and water without solids can be drained from the bottom of the filter. As in other multi-layered filters, the hydraulic conductivity of the filter unit is determined by the layer having the lowest conductivity. Filter-material with a low hydraulic conductivity cannot be used for such a filter.
U.S. Pat. No. 5,632,889 (THARP) discloses a filter cartridge for separating liquid hydrocarbons from water. The filter cartridge comprises perlite particles which have been treated with a silicone. Runoff water may percolate through a body of particulate perlite so that hydrocarbons are absorbed by the perlite particles and pure water can be drained from the bottom of the filter. One disadvantage of this filter unit is that it is confined to perlite particles and therefore not useful for removing pollutants, which are not absorbed by perlite. Furthermore, as pointed out above there is a potential risk that larger pores are created through the layer, so that part of the liquid bypasses the filter without coming into contact with the inside of the perlite particles.
U.S. Pat. No. 4,761,232 (POREX TECHNOLOGIES CORP.) discloses a macroporous polyethylene substrate defining a network of interconnecting macropores and a microporous matrix of polyvinyl chloride, which completely fills the network of macropores. This filter has many features in common with the perlite filter disclosed in U.S. Pat. No. 5,632,889 since it also consists of a network of intermingled macro and micropores. One drawback of such a filter layout is that the microporous matrix cannot be removed independently from the macroporous substrate, when the absorptive capacity of the former is exhausted. Furthermore, the whole filter has to be exchanged as soon as the absorptive capacity of the micropores has been exhausted in just one location.
U.S. Pat. No. 5,980,761 (BOISSIE ET AL) discloses a filter unit being cylindrical or frustoconical and containing pozzolan as a particulate filter material. The water to be filtered may pass in a direction both vertically through the unit and in a horizontal direction. In the case of two or more different filter materials, the water passes sequentially through the layers. Thus the filter is a special embodiment of a traditional multi-layered filter.
Definitions
Convective layer—is defined according to the present invention as a layer designed to conduct fluid to be changed, either in substance composition or otherwise. It is an open structure that allows the fluid to flow through the filter unit. In the layout of the convective layer there will be a main direction of flow along one axis. In most embodiments, the convective layer is given a high hydraulic conductivity and a flat shape (e.g. in the shape of a sheet having a defined maximal thickness) in order to allow for effective transfer of substances or other features between the convective layer and the receiving layer. The axis of main direction of flow in the convective layer is perpendicular to the smallest dimension of the layer (i.e. the height of a sheet), and thus parallel to either the length or the width dimensions of the layer.
Receiving layer—is defined as a layer adjacent to at least one convective layer and designed to receive substance(s) from the fluid in this convective layer without being percolated by the fluid. Additionally, the receiving layer can be designed to enrich a fluid in the convective layer with substance(s). The receiving layer can be made to accumulate substance(s), transform or degrade substance(s), transfer substance(s), or release substance(s) or to otherwise affect the composition or other characteristics, e.g. temperature, of the fluid in the convective layer. The receiving layer can itself consist of several layers with different functions. The fluid in the receiving layer can either be the same fluid as in the adjacent convective layer, or another fluid. The fluid in the receiving layer can be stagnant, in which case the receiving layer is a stagnant layer, or mobile. If the fluid in the receiving layer is mobile it flows at a rate different from the flow rate of the fluid in the convective layer, and/or the main direction of flow in the receiving layer is different from the main direction of flow in the convective layer.
Filtering material—is used to describe a material placed within the receiving layer. The functioning of the filtering material is to to hold back, transform or degrade substance(s) from the fluid in the convective layer, or to support transfer of substance(s) from the convective layer. Additionally, the filtering material can be used to provide substance(s) to enrich or feed a fluid in the convective layer.
Substance(s)—is used to describe target substances, i.e. substances of interest regarding the fluid in the convective layer, either substances to be removed from the fluid, e.g. contaminants or pollutants, or substances to enter the fluid. Substances comprises any organic and/or inorganic solute in continuous or discontinuous phase, any organic and/or inorganic particle irrespective of size, and any organism, that can be transferred between the fluid in the convective layer and the receiving layer, and for which a receiving layer can be designed.
Mixing-zone mass flow—is used to describe the phenomenon that a fraction of the fluid in the convective layer change place with a fraction of fluid in the receiving layer due to turbulent occurring at the interface between convective layer and receiving layer.
Mixing-zone—is used to describe the area where mixing-zone mass flow takes place.
Sandwich filter—is used to describe all layers of a device or a part of a device comprising at least two layers comprising at least one receiving layer and at least one convective layer. A sandwich filter can comprise different numbers of receiving layers and convective layer e.g. two receiving layers and one convective layer.
Stacks of sandwich filters can be stacks of similar sandwich filters or stacks of different sandwich filters. When sandwich filters are stacked two receiving layers or two convective layers or one receiving layer and one convective layer becomes the adjacent layers of the two sandwich filters.
Filter unit—is used to describe an entire unity of a device based on the invention, thus a filter unit comprises at least one receiving layer and at least one convective layer. A filter unit can comprise any number of sandwich filters. Preferred is when a liquid and/or gas impermeable material surrounds a filter unit, although this is not a requisite. Two of more filter units can be combined or connected e.g. to obtain specific fluid treatment characteristics. Also an impermeable or semi-permeable layer e.g. an impermeable or semi-permeable membrane can be placed between two or more sandwich filters of the filter unit.